Here's one that is only kind of mentioned, there are actually different altitudes. If you use ADS-B data you will only get the barometric altitude which is not calibrated to the ground pressure level. For example if you watch ADS-B data of flights into Denver it appears that every aircraft is crashing down ~5000ft during landing.
Above ground level, and above sea level are two easily confusable ways of reporting altitude. Reporting a value above sea level on ADSB so that it doesn't change is probably the right thing to do, especially since airfields report their local pressure and have their elevation recorded.
Expecting every runway to be above sea level is another falsehood. AMS has runways 3 meters below sea level, and I'm sure there's even lower ones on the east side of Africa.
